Method for current sensing in switched dc-to-dc converters

ABSTRACT

A method for sensing the supply current of a switched DC-to-DC converter is discussed. The method sensing a first voltage that is proportional to the supply current, wherein the first voltage has first noise; outputting a second voltage that is based on the first voltage, and wherein the second voltage has second noise that is smaller than the first noise; and comparing the second voltage to a reference voltage to provide an indication of the supply current. According to the systems and methods disclosed herein, accurate current sensing is provided.

This application is a Continuation of U.S. application Ser. No.11/330,883, filed Jan. 11, 2006, which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to electronic circuits, and moreparticularly to a system for sensing the supply current, for example ina switched DC-to-DC converter.

BACKGROUND OF THE INVENTION

DC-to-DC converters are well known. A DC-to-DC converter is a devicethat receives a DC input voltage and produces a DC output voltage. Theoutput voltage is typically produced at a different voltage level fromthe input voltage. DC-to-DC converters can produce high voltage usablefor low power applications including supplying power to mobile devicessuch as cellular phones and laptop computers.

Sensing supply current is one task that a DC-to-DC converter performsduring a voltage conversion process. Supply current sensing in aDC-to-DC converter is generally implemented by sensing the voltagegenerated by the supply current flowing through a sense resistor.Sometimes the voltage across a switch is directly sensed where the “on”resistance of the switch is used as a sense resistor. Because the sensevoltage is based on the supply current, the behavior of the supplycurrent (e.g., glitches) will be reflected in the behavior of the sensevoltage. This is problematic, because the resulting sense voltage may bevery noisy (i.e., having a high ripple). As a result, current sensing ofthe conventional DC-to-DC converter is performed with poor accuracy.

Accordingly, what is needed is an improved system for sensing the supplycurrent of a switched DC-to-DC converter. The present inventionaddresses such a need.

SUMMARY OF THE INVENTION

A system for sensing the supply current of a switched DC-to-DC converteris disclosed. The system includes a first circuit that senses a firstvoltage that is proportional to the supply current, wherein the firstvoltage has first ripple; a second circuit coupled to the first circuit,wherein the second circuit outputs a second voltage that is based on thefirst voltage, and wherein the second voltage has a second ripple thatis smaller than the first ripple; and a third circuit coupled to thesecond circuit, wherein the third circuit compares the second voltage toa reference voltage to provide an indication of the supply current.

According to the system disclosed herein, the system provides accuratecurrent sensing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional buck DC-to-DC converterwhere current sensing is performed using a sense resistor in series witha switch.

FIG. 2 is a block diagram of the conventional buck DC-to-DC convertersimilar to the DC-to-DC converter of FIG. 1, except that the currentsensing is performed using an “on” resistance of a switch.

FIG. 3 shows the behavior of the current sensing performed by theDC-to-DC converter of FIG. 1.

FIG. 4 is a schematic diagram of a DC-to-DC converter in accordance withone embodiment of the present invention.

FIG. 5 is a schematic diagram of a DC-to-DC converter in accordance withanother embodiment of the present invention.

FIG. 6 shows the behavior of voltages V_(sense1) and V_(sense1) for theDC-to-DC converter shown in FIG. 4 when the DC-to-DC load is changedfrom 100 mA to 1 A, in accordance with the present invention.

FIG. 7 is a schematic diagram of a DC-to-DC converter in accordance withanother embodiment of the present invention.

FIG. 8 is a schematic diagram of a DC-to-DC converter 800 in accordancewith another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to electronic circuits, and moreparticularly to a system for sensing the supply current of a switchedDC-to-DC converter. The following description is presented to enable oneof ordinary skill in the art to make and use the invention, and isprovided in the context of a patent application and its requirements.Various modifications to the preferred embodiment and the genericprinciples and features described herein will be readily apparent tothose skilled in the art. Thus, the present invention is not intended tobe limited to the embodiments shown, but is to be accorded the widestscope consistent with the principles and features described herein.

A system in accordance with the present invention for sensing the supplycurrent of a switched DC-to-DC converter is disclosed. The systemincludes a first circuit that senses a sense voltage, which isproportional to the supply current. The sense voltage has ripple thatmay be relatively high. The system also includes a second circuit thatoutputs a second voltage that is based on the sense voltage, where thesecond voltage has a ripple that is substantially smaller than theripple of the sense voltage. The system also includes a third circuitthat compares the second voltage to a reference voltage to provide anindication of the supply current. Because the second voltage has asmaller ripple than that of the sense voltage, the circuit can sense thesupply current with better accuracy. To more particularly describe thefeatures of the present invention, refer now to the followingdescription in conjunction with the accompanying figures.

Although the present invention disclosed herein is described in thecontext of switch DC-to-DC converters, the present invention may applyto types of converters, and still remain within the spirit and scope ofthe present invention.

FIG. 1 is a block diagram of a conventional buck DC-to-DC converter 50where current sensing is performed using a sense resistor 52 in serieswith a switch 54. The DC-to-DC converter 50 also includes a comparator56, an OR gate 58, a delay block 60, a switch 62, an inductor 64, and acapacitor 66. FIG. 2 is a block diagram of the conventional buckDC-to-DC converter 70 similar to the DC-to-DC converter 50 of FIG. 1,except that the current sensing is performed using an “on” resistance ofthe switch 54. With both DC-to-DC converters 50 and 70, a sense voltageV_(sense) is compared to a pre-defined reference voltage V_(r). Thereference voltage V_(r) may represent, for example, the maximum currentallowed to flow through the switch 54.

FIG. 3 shows the behavior of the current sensing performed by theDC-to-DC converter 50 of FIG. 1. Referring to both FIGS. 1 and 3together, the two switches 54 and 62 alternatively turn on and off suchthat the signals dh and dl behave like a two-phase clock signal, theduty cycle of which defines the value of the DC-to-DC converter outputvoltage V_(out). The switch 54 is controlled by the signal dh.Accordingly, when the signal dh is low, the switch 54 turns on, and thesupply current I_(dcdc) flows from the voltage supply V_(dd) to theoutput voltage V_(out). The sense resistor 52 is connected between thevoltage supply V_(dd) and the switch 54. When the switch 54 is on, thesupply current I_(dcdc) flows through the resistor 52 and generates asense voltage V_(sense), which is proportional to and thus represents ameasure of the supply current I_(dcdc). When the switch 54 is off, nocurrent flows through the resistor 52 (i.e., I_(dcdc)=0) and the sensevoltage V_(sense) is equal to the supply voltage V_(dd). By comparingthe sense voltage V_(sense) with a proper reference voltage V_(r), theoutput is only valid when the switch 54 is on. That is, the output ofthe DC-to-DC converter 50 must be gated with the same signal used toturn on the switch 54; i.e., the output of the DC-to-DC converter 50must be ORed with the signal dh. A problem with the DC-to-DC converter50 is that glitches can occur in the current due to direct path current(i.e., non-ideal switching of the switches 54 and 62) as well as in thesense voltage V_(sense). Consequently, the output of the comparator canchange state, even if the current threshold determined by the voltageV_(r) is not reached. This is shown in FIG. 3 where glitches on thesupply current I_(dcdc) generate glitches on the sense voltageV_(sense), which makes the output of the DC-to-DC converter 50 changestate. To avoid this behavior, the OR gate 58 is controlled by signaldh1 instead of signal dh. Signal dh1 (generated through delay block 60)has a delayed falling edge with respect to dh, and thus, as shown inFIG. 3, the comparator changes its state only if the sense voltageV_(sense) becomes lower than the threshold V_(r). Glitches have noeffect on the comparator output.

The DC-to-DC converter 70 of FIG. 2, where the current sensing isperformed by sensing the voltage drop across switch 54, has the sameproblems as the DC-to-DC converter 50 of FIG. 1. The problem with theDC-to-DC converters 50 and 70 is that the sense voltage elements have avery high ripple, as shown in FIG. 3. Consequently, this makes itdifficult to perform accurate current sensing. Assuming, for example,that the current threshold at 1 A and the sense resistor 52 0.1Ω, theripple voltage is about 100 mV. Both circuits of FIGS. 1 and 2 have thisproblem.

In accordance with the present invention, a DC-to-DC converter sensesDC-to-DC current by sensing the voltage drop across a sense resistor oracross a switch by providing to the comparator a clean sense voltageV_(sense) (i.e., a sense voltage with a very small ripple).

FIG. 4 is a schematic diagram of a DC-to-DC converter 400 in accordancewith one embodiment of the present invention. The DC-to-DC converter 400includes a switched capacitor circuit 402 that has switches 404 and 406and capacitors 408 and 410. The DC-to-DC converter 400 also includesswitches 412 and 414, a sense resistor 416, an inductor 418, a capacitor420, a delay block 422, and a comparator 424. In one embodiment, theswitches 412 and 414 are PMOS and NMOS transistors, respectively.

In operation, the supply current I_(dcdc) of the DC-to-DC converter 70is sensed through the sense resistor 416. The switches 412 and 414 andthe sense resistor 416 sense the sense voltage V_(sense), which isproportional to and thus representative of the supply current I_(dcdc).The sense voltage V_(sense) is input to the switched capacitor circuit402, which generates voltage V_(sense1) that is used as input for thecomparator 424. The voltage V_(sense1) has a very small ripple. A signaldh1 controls the switches 404 and 406. The signal dh1 is used instead ofdh in order to avoid glitches on the sense voltage V_(sense).

When the switch 412 turns on, the switch 404 closes and the switch 404opens. The capacitor 408 is then charged such that the capacitor 408stores a charge that has a voltage that matches the sense voltageV_(sense). The capacitor 410 remains floating, making voltage V_(sense1)constant. The input resistance of the comparator 424 is assumed to beinfinite. When the switch 412 turns off, the switch 404 opens and theswitch 406 closes. When the switch 406 closes, the capacitors 408 and410 share their charges. In other words, the capacitor 408 charges thecapacitor 410 such that the capacitor 410 stores a voltage that matchesthe sense voltage V_(sense), thereby biasing the voltage V_(sense1) suchthat the voltage V_(sense1) is based on the sense voltage V_(sense) andthus reflects or represents the supply current I_(dcdc). The voltageV_(sense1) then represents the current I_(dcdc) flowing from the supply.Accordingly, when the load current changes, the voltages V_(sense) andV_(sense1) change as well. The transmit time depends on the ratiobetween the values of the capacitors 408 and 410.

In accordance with the present invention, due to the charge sharingaspect of the capacitors 408 and 410, high values of the capacitor 410relative to the capacitor 408 allow only a very small ripple on thevoltage V_(sense1) but increase the transient time with respect to aload variation. The size of the switches and capacitors are optimized tominimize charge injection effects and to minimize the ripple on thevoltage V_(sense1). As a result, the fixed load condition is very small,thus allowing accurate current sensing.

The comparator 424 compares the voltage V_(sense1) to the referencevoltage V_(r), representing a threshold value of the supply current. Thethreshold value may be, for example, the maximum supply current allowedto flow through the switch 412. Accordingly, comparing the voltageV_(sense1) to the reference voltage V_(r) provides an indication of thesupply current I_(dcdc) (e.g., whether the supply current I_(dcdc) isabove or below a threshold current represented by the reference voltageV_(r)).

FIG. 5 is a schematic diagram of a DC-to-DC converter 500 in accordancewith another embodiment of the present invention. The DC-to-DC converter500 is similar to the DC-to-DC converter 400 of FIG. 4, except that inthe DC-to-DC converter 500, the DC-to-DC current I_(dcdc) is sensedthrough an “on” resistance of the switch 412. In operation, the DC-to-DCconverter 500 functions similarly to the DC-to-DC converter 400.

FIG. 6 shows the behavior of voltages V_(sense) and V_(sense1) for theDC-to-DC converter 400 shown in FIG. 4 when the DC-to-DC load is changedfrom 100 mA to 1 A, in accordance with the present invention. This hasbeen obtained by using C1=0.25 pF and C2=1 pF. Note that the sensevoltage V_(sense) has very high ripple (i.e., more than 100 mV) comparedto the very small ripple of the voltage V_(sense1) (i.e., a few mV).Because this ripple is very small, current sensing can be performed withmore accuracy.

FIG. 7 is a schematic diagram of a DC-to-DC converter 700 in accordancewith another embodiment of the present invention. The DC-to-DC converter700 is similar to the DC-to-DC converter 400 of FIG. 5, except for theswitched capacitor circuit 702. The switched capacitor circuit 702 ofthe DC-to-DC converter 700 includes switches 704, 706, 708, and 710, andincludes capacitors 712 and 714. Also, one node of the capacitor 712 isconnected to V_(dd) (instead of to ground), and one node of thecapacitor 714 is connected to a reference bandgap voltage V_(bg)(instead of to ground).

Referring briefly to FIG. 4, if the supply current I_(dcdc) to be sensedis very low, the voltage V_(sense1) will be slightly below the supplyvoltage V_(dd). If the value of the sense resistor 416 is 0.1Ω and thecurrent I_(dcdc) to be sensed is 100 mA, the voltage V_(sense1) will beabout 10 mV below the supply voltage V_(dd), and the comparator 424 canthus work with a very-high-input common-mode voltage.

Referring again to FIG. 7, the DC-to-DC converter 700 allows a change ofthe common-mode voltage of the voltage V_(sense1). When the switch 722turns on, the capacitor 712 is charged to the voltage across theresistor 726. When the switch 722 turns off, the capacitor 712 sharesits charge with the capacitor 714, which has one node connected to thebandgap voltage V_(bg). Assuming that the sense resistor 726=0.1Ω, thebandgap voltage V_(bg)=1.2V, and the supply current I_(dcdc) to besensed is 100 mA, the resulting voltage V_(sense1) will be 10 mV belowthe bandgap voltage V_(bg) (i.e., 1.19V). This means that the comparator734 operates with a good common-mode input voltage relative to theDC-to-DC converters 400 and 500 of FIGS. 4 and 5, respectively.

In an alternative embodiment, a ratiometric common-mode voltage can beused instead of the bandgap voltage V_(bg). This would guaranty that theDC-to-DC comparator 734 operates with its best common mode inputvoltage, thus minimizing the input offset.

FIG. 8 is a schematic diagram of a DC-to-DC converter 800 in accordancewith another embodiment of the present invention. The DC-to-DC converter800 is similar to the DC-to-DC converter 700 of FIG. 7, except that inthe DC-to-DC converter 800, the DC-to-DC current I_(dcdc) is sensedthrough an “on” resistance of the switch 822.

According to the system disclosed herein, the present invention providesnumerous benefits. For example, it provides accurate current sensingwith low current overhead.

A system in accordance with the present invention for sensing the supplycurrent of a switched DC-to-DC converter has been disclosed. The systemincludes a first circuit that senses a sense voltage, which isproportional to the supply current. The sense voltage has ripple thatmay be relatively high. The system also includes a second circuit thatoutputs a second voltage that is based on the sense voltage, where thesecond voltage has a ripple that is substantially smaller than theripple of the sense voltage. The system also includes a third circuitthat compares the second voltage to a reference voltage to provide anindication of the supply current. Because the second voltage has asmaller ripple than that of the sense voltage, the system can sense thesupply current with better accuracy.

The present invention has been described in accordance with theembodiments shown. One of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments, and thatany variations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe appended claims.

1. A method for sensing a supply current, comprising: sensing a firstvoltage that is proportional to the supply current and includes firstnoise; outputting a second voltage that is based on the first voltageand includes second noise that is smaller than the first noise; andcomparing the second voltage to a reference voltage to provide anindication of the supply current.
 2. The method of claim 1, whereinoutputting a second voltage includes, during a first phase, storing afirst charge having a first charge voltage based on the first voltage.3. The method of claim 2, wherein outputting a second voltage includes,during a second phase, storing a second charge having a second chargevoltage based on the first charge voltage.
 4. The method of claim 3,wherein sensing a first voltage includes controlling a switchedcapacitor circuit using a delay signal.
 5. The method of claim 4,wherein outputting a second voltage includes switching a first switch tocharge a first capacitor and switching a second switch to charge asecond capacitor based on the delay signal.
 6. The method of claim 5,wherein switching the first switch includes closing the first switchonly when the second switch is open.
 7. The method of claim 6, whereinthe delay signal operates to correct for a transit delay between thefirst capacitor and the second capacitor.
 8. The method of claim 1,wherein outputting a second voltage includes switching a first switch tocharge a first capacitor and switching a second switch to charge asecond capacitor.